1. Field of the Invention
The present invention relates to an image processing device and an image processing method for correcting an image data item using an offset data item in a radiation image processing field.
2. Description of the Related Art
In a radiograph apparatus in which a radiation detector is mounted, in order to reduce noise that occurs due to the characteristics of the radiation detector, noise correction is performed using an offset data item. The offset data item is a data item that is read from the radiation detector when radiation irradiation is not performed, and is referred to as a dark current or a dark image. A noise correction method is performed by subtracting the offset data item from an image date item. In other words, the offset data item obtained by reading only noise is subtracted from an image data in which noise exists, thereby performing noise correction.
Japanese Patent Laid-Open No. 2006-267093 discloses a method for reading an offset data item shown in FIG. 5. In the method shown in FIG. 5, each time that an image data item is read, an offset data item is read before and after the image data item is read. An offset data item that is read before an image data item is read is referred to as a “pre-offset data item”. An offset data item that is read after the image data item has been read is referred to as a “post-offset data item”. For example, O2 denotes a pre-offset data item for an image data item I2, and O3 denotes a post-offset data item for the image data item I2. X1, X2, and X3 denote each a time at which X-ray irradiation is performed.
Typically, a noise correction process using an offset data item is performed using a post-offset data item. The reason for this is that an afterimage of an image of the previous frame exists in a pre-offset data item, and, when the pre-offset data item is subtracted from an image data item, the image data item is influenced by the afterimage.
In other words, referring to FIG. 5, to correct the image data item I2, a subtraction process represented by (I2−O2) may be performed. However, in such a case, because an afterimage of an image data item I1 exists in the pre-offset data item O2, the afterimage of the image data item I1 influences the image data item I2, although the influence is small. In contrast, a subtraction process represented by (I2−O3) may alternatively be performed. In this case, only an afterimage of the image data item I2 exists in the post-offset data item O3. Accordingly, the negative influence of the afterimage is smaller and the quality of an image is higher than that of an image corrected by the subtraction process represented by (I2−O2).
However, in the correction process using a post-offset data item, a time taken to obtain a post-offset data item is necessary after an image data item has been obtained. Thus, a time taken from when the image data item is obtained to when the image data item is displayed is longer than that taken in a case in which the image data item is corrected using a pre-offset data item.
Additionally, as described above, when the correction process using a pre-offset data item is performed, a time taken until an image data item is displayed is short. However, the quality of an image is lower than that of an image which is corrected using a post-offset data item.
In addition, in a case of image diagnostic in the medical field, a higher-quality image frequently is necessary so that a small tumor or the like can be found by checking the image. Furthermore, in a moving-image-pickup apparatus using radiation, a current or voltage value that is necessary in order to generate radiation while an image is being picked up is controlled using image analysis. Accordingly, in order to perform the image analysis with a high accuracy, it is important to obtain a high-quality image.
In the medical field, in particular, displaying of an image in real time is advantageous to a case in which the moving-image-pickup apparatus using radiation is used in an operation. For example, when a manipulation in which a catheter is inserted into a human body is performed, while a doctor is irradiating the human body with radiation, an image data item obtained as a result of the irradiation is displayed on a monitor. The doctor continues inserting the catheter while checking the displayed image. In such a case, when there is a time difference between insertion of the catheter and corresponding display of the image, there is a risk that the doctor moves the catheter in a wrong direction. Thus, displaying of the image in real time is important.